Poly(arylene sulfides) (hereinafter abbreviated as “PASs”) represented by poly(phenylene sulfide) (hereinafter abbreviated as “PPS”) are engineering plastics excellent in heat resistance, chemical resistance, flame retardancy, “mechanical strength, electrical properties, dimensional stability and the like. The PASs are commonly used in a wide variety of fields such as electrical and electronic equipments and automotive equipments because they can be formed or molded into various kinds of molded or formed products, films, sheets, fibers, etc. by general melt processing techniques such as extrusion, injection molding and compression molding.
As a typical production process of a PAS, is known a process, in which a sulfur source is reacted with a dihalo-aromatic compound in an organic amide solvent such as N-methyl-2-pyrrolidone (hereinafter abbreviated as “NMP”). As the sulfur source, is mainly used an alkali metal sulfide. There is also known a method of using, as the sulfur source, an alkali metal sulfide formed by reacting an alkali metal hydrosulfide with an alkali metal hydroxide in situ.
The polymerization reaction of the sulfur source with the dihalo-aromatic compound is a desalting polycondensation reaction to produce a great amount of a salt (i.e., an alkali metal halide) such as NaCl after the reaction. There have therefore been proposed methods for removing a salt such as NaCl by washing the PAS obtained by the reaction with a washing liquid such as water, an organic solvent, a mixture of water and an organic solvent or water containing a surfactant (for example, Japanese Patent Publication No. 6-86528, Japanese Patent Publication No. 6-57747, Japanese Patent Application Laid-Open No. 4-139215, Japanese Patent Publication No. 4-55445 and Japanese Patent Application Laid-Open No. 10-265575).
On the other hand, There have been proposed production processes of a poly(arylene sulfide), in which a sulfur source and a dihalo-aromatic compound are polymerized in the presence of an alkali metal hydroxide (for example, Japanese Patent Application Laid-Open No. 2-302436, Japanese Patent Application Laid-Open No. 5-271414, Japanese Patent Publication No. 6-51792, Japanese Patent Application Laid-Open No. 2001-181394, Japanese Patent Application Laid-Open No. 2-160834 and Japanese Patent Publication No. 6-51793). According to these production processes, however, it is difficult to set conditions for stably performing the polymerization reaction. In addition, since a great amount of the alkali metal hydroxide is used in these production processes, it is difficult to inhibit side reactions.
When the sulfur source and the dihalo-aromatic compound are polymerized in the presence of the alkali metal hydroxide in the organic amide solvent, the organic amide solvent such as NMP is reacted with the alkali metal hydroxide by heating to produce a compound containing a nitrogen atom as impurities. For example, when NMP reacts with sodium hydroxide (NaOH), NMP is subjected to ring opening to produce sodium methylaminobutanoate [(CH3)NH—CH2—CH2—CH2—COONa]. Sodium methylaminobutanoate reacts with the dihalo-aromatic compound that is a monomer component. For example, sodium methylaminobutanoate reacts with p-dichlorobenzene to produce sodium chlorophenylmethylaminobutanoate.
Such a nitrogen atom-containing compound is difficult to be sufficiently removed even when the PAS is purified in a washing step after the polymerization. For example, even when the PAS obtained in the polymerization step is washed with an organic solvent or water, thereby removing by-products such as NaCl, and the content of a low-molecular weight component extracted by Soxhlet extraction with chloroform is reduced to at most 5% by weight, preferably at most 4% by weight, more preferably at most 3% by weight, the nitrogen atom-containing compound remains as impurities.
As a result, the resulting PAS has involved a problem that when the PAS is injection-molded or extruded, the impurities adhere to a mold or die. Since stain of the mold or die, which is caused by such a nitrogen atom-containing compound, adversely affects the quality of a molded or formed product, the mold or die requires frequent cleaning.